This invention is directed generally to sensing nearby targets. More particularly, this invention involves the use of continuous wave (CW), radio frequency (RF) signals for measuring spatial properties or merely detecting the presence of suitably-positioned metallic or non-metallic targets, in a manner that is tolerant of harsh environments.
Conventional proximity sensing uses inductive switches to detect metal objects at different distances. An electromagnetic field generated by the sensor induces eddy currents on the surface of a metal target, which in turn modifies the sensor's oscillator voltage. This change indicates the presence of the target. These sensors are quite limited in that they can only sense conductive materials and the sensing distance is dependent on sensor size. Moreover, the sensor performance is susceptible to strong external magnetic fields.
Another category of sensors use light-based technology. A familiar application of light-based sensing is the laser speed detector used by police to determine the speed of a vehicle. These detectors measure how long it takes a pulse to travel from the sensor module to the target and back again to determine distance, and use two such measurements to calculate speed. Although sensors of this type can detect the presence and range of a target, they are not well suited for very short-range operation in harsh environments.
By utilizing electromagnetic energy, RF sensing offers greater versatility, particularly with regard to environmental tolerance. Microwave energy, for example, can easily penetrate dirt, oils, rain, fog, or snow. Microwave detection systems have been in use for a number of years, but they have substantial drawbacks. Others use pulse signals to measure time of flight, but this approach works poorly for short distances. Fast, accurate pulse measurements at close range require the use of extremely narrow pulses, which are particularly very difficult to generate, transmit, detect and correlate. Still others require digital circuitry, multiple antennas, modulation schemes, phase shifting circuitry or frequency division circuitry, which often adversely affects the cost, form factor, durability or versatility of the sensor. Such disadvantages have prevented RF sensing from being viable in numerous applications.